The present invention relates to a spectral light division and recombination configuration as well as a process for the spectrally selective modulation of light.
Definitions:
                Light,        visible light: light with maximum energy in the spectral range 380 nm–720 nm        red light: light with maximum energy in the spectral range 580 nm–780 nm, in particular in the spectral range 600 nm–680 nm        green light: light with maximum energy in the spectral range 490 nm–605 nm, in particular in the spectral range 500 nm–600 nm        blue light: light with maximum energy in the spectral range 380 nm–510 nm, in particular in the spectral range 420 nm–500 nm        yellow light: light with maximum energy in the spectral range 475 nm–605 nm, in particular at 482 nm+3 nm        white light: light with red, blue and green light components        
linearly polarized light: light whose electric field vector oscillates in a plane.                reflective light        valve: image producing element which operates in reflection, for example on the basis of liquid crystals with polarization rotation (rLCD).        bright state rLCD: With reflection of light on pixels of the reflective light valve, the polarization is rotated by an odd-numbered multiple of 90°.        dark state rLCD: With reflection of light on pixels of the reflective light valve the polarization is maintained.        F number: Variable describing the angle opening-out of an illumination optics system. It is given by the reciprocal value of the twofold numerical aperture NA, wherein NA=n sin α, and n is the refractive index of the medium, α one half of the aperture angle of the illumination cone. Therefore, the smaller the F number, the wider the aperture angle. Typical F numbers are in the range from 5 to 2.5; F numbers of 2 down to 1.4 are also used.        
Optical light division and recombination configurations are used primarily in projectors in order to divide white light into red, green and blue light or to recombine the latter in order to form white light.
It is known to divide, by means of such a configuration in projection apparatus, white light into light of said three spectral ranges, to modulate each via light valves operating in transmission, such as in particular LCD light valves, to be image producing and subsequently to recombine the modulated light of the three spectra to form an imaging beam. The light valves, as the image-producing structural elements, comprise a multiplicity of individually drivable pixels. Their number yields therein the resolution according to the formats EVGA, SVGA, EGA, XGA, etc. When using light valves operating in transmission a lower limitation of the pixel size exists which can only be overcome with difficulty due to the printed conductor tracks and the drive electronics. In addition, with a decrease of the pixel size, the optical aperture per pixel decreases.
The present invention, in contrast, relates to a process for the spectrally selective modulation of light by means of light valves operating in reflection. The optical light division and recombination configuration, to which the invention relates further, consequently is preferably used in combination with light valves operating in reflection for the formation of a configuration according to the invention for the optical light division, spectrally selective modulation and subsequent recombination.
When using light valves operating in reflection the restrictions regarding light valves operating in transmission cease to apply. Controlled light valves operating in reflection do not rotate in the dark state of a pixel the polarization of the light reflected thereon with respect to that of the incident light, while in the bright state the polarization of the reflected light is rotated by 90° with respect to the polarization of the incident light.
In a process of the above described type, or an optical light division and recombination configuration of the above type, it is necessary, on the one hand, to ensure that white light is divided into light of said three spectra, and light of said three spectra, after reflective modulation and modulation-dependent or drive-dependent polarization rotation, is combined to white light, therewith, on the other hand, light from pixels operated in the bright state should be recombined to form a first light beam—the imaging beam—, light of pixels operated in the dark state not emerging on said beam, therewith in particular, should be recombined to form a second light beam—the dark reflection beam, wherein both said beams should leave the light division and recombination configuration in different directions. The first is preferably, and in view of the use on a projection arrangement, guided to its imaging optics system; the latter should, for example or preferably, be guided back to the white light source.
Previously known solutions for processes, and for optical light division and recombination configurations of the above type, can be divided into formulations using glass plates and formulations with solid glass bodies. In both cases, again, geometries can be differentiated which operate on the basis of angular beam deflections of 45° or 90°, and those operating on the basis of angles which differ from beam deflections of 45° or 90°.
Configurations realized with angles of 45° are described, for example, in DE 40 33 842, which describes a rectangular parallelepiped structural element composed of discrete prisms with dichroic layers. Such a structural element is customarily referred to as an X-cube. With respect to such X-cubes and their uses, further reference is made to U.S. Pat. No. 2,737,076, U.S. Pat. No. 2,754,718, JP 7-109443, U.S. Pat. No. 5,098,183, EP A 0 359 461, as well as WO98/20383 by the same applicant as the present application.
Angles deviating from 45° as well as solid glass bodies are used, for example, according to U.S. Pat. No. 3,203,039 which lead to configurations generally known as “Philips prisms”.
Furthermore are known diverse combinations of dichroic splitter configuration plates—spectrally selective splitters—combined with prism configurations, thus, for example, from U.S. Pat. No. 3,945,034, or combined with X-cubes.
For a realization form known under the designation 3PBS system, reference can be made to R. L. Melcher “High Information—Content Protection Display Based on Reflective LC on Silicon Light Valves”, SID 98 Digest, pp. 25–28, 1998.
As has been mentioned, in spectral light division and recombination light of different polarization states must be handled after the reflective modulation.
An X-cube configuration, if appropriate in combination with a polarization-selective beam-splitting configuration, such as is described for example in said WO98/20383, presupposes dichroic layer systems, which have minimum polarization effects since they are acted upon by light of different polarization, which should only be handled with spectral selectivity.
In this respect reference is made to A. Thelen “Nonpolarising interference films inside a glass cube”, Appl. Optics Vol. 15, No. 12, December 1976.
Regarding problems concerning the dark-state transmission characteristics of light division and recombination techniques operating with reflective light valves, reference is made to A. E. Rosenbluth et al.: “Contrast properties of reflective liquid crystal light valves in projection displays”, IBM Journal of Research and Development, Vol. 42, No. 3/4 May/July, pp. 359–383,1998. A solution of these problems is only possible with considerable design and fabrication expenditures.
Realizations according to the above listed Philips prisms or similar approaches are based, as a rule, on total internal reflection at one or several air gaps. Apart from the difficulties in the production of such air gaps, the total reflection is the limiting factor for the F numbers possible in practice. In addition, such systems are very sensitive with respect to residual reflections which, for example, can occur if the transitions from one optical refractive index to the other are not optimally matched and which as ghosts can ‘haunt’ the system under diverse directions.
In the case of approaches toward a solution using solid glass bodies, the problem of the mechanical birefringence in the glass or the substrate material must be taken into consideration whereby the polarization of the light is rotated uncontrollably with stochastic distribution. A contrast loss is generated in the process which is not constant over the illuminated surface.
Structural parts in which dichroic plates in the imaging ray path are used, are, on the other hand, afflicted with the problem that an astigmatism is generated through the plates. The high requirements made of the planity of such substrates places limits on the transition to thinner substrates in which this astigmatism would become negligibly small.